Magnetic field sensors are commonly used in modern systems to measure the magnetic field, current, position, direction, and many other physical parameters. In the prior art, there are many different types of sensors for measuring magnetic field, and these commonly use the Hall Effect, an anisotropic magnetoresistance (AMR), or giant magnetoresistance (GMR) elements.
Hall Effect sensors generally have low sensitivity, and thus often use a flux concentrator to increase the sensitivity. Flux concentrators increase the size and weight of the sensor, and can decrease the linearity of the sensor. Moreover, Hall elements generally have high power consumption. Although AMR elements have much higher sensitivity than Hall elements, they suffer from narrow linear range, and they need to be reset using a set/reset coil. The reset operation, resulting increases the complexity of the manufacturing process; the coil increases the size of the sensor; and, the power consumption also increased by the reset operation. Multilayer GMR sensor elements have higher sensitivity than AMR, but their linear range is low, and unless they are biased by a permanent magnet, the response curve of the can only provide a unipolar measurement of the magnetic field gradient, a bipolar magnetic field gradient cannot be measured.
Magnetic tunnel junction (MTJ) elements in recent years have been gaining acceptance as magnetoresistive sensors in industrial applications. They are multilayer devices that utilize of the tunnel magnetoresistance effect (TMR) for measuring the magnetic field, and these elements posses a larger rate of resistance change than AMR or GMR elements. Compared to the Hall Effect sensors, MTJ elements have superior temperature stability, higher sensitivity, lower power consumption, better linearity, and no need for flux concentrators; with respect to AMR sensors they have improved temperature stability, higher sensitivity, wider linear range, and no need for a set/reset coil structure; Compared to GMR sensors they have better temperature stability, higher sensitivity, lower power consumption, and wider linear range.
Although MTJ elements have high sensitivity, when used to detect weak field gradients the MTJ element can be disturbed by strong magnetic fields, and the high sensitivity MTJ elements are not easily amenable to low-cost mass production. The sensor yield depends on the offset value output from MTJ magnetoresistive element constituting the magnetoresistive bridge, because MTJ elements are often difficult to match on the different arms of the bridge. The manufacturing process for fully single-chip MTJ sensor bridges is very complicated.